[FarSe.]

pawn Код:

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if(newkeys == 16)//F,from what i remember
    {
    OnPlayerCommandText(playerid,"/enter");
    }
    return 1;
}

Yes,This maybe will lag your server,because everytime when he press ENTER,the whole OnPlayerComandText+'/enter'/ command will executing ..